Hacktivism—from the terms “hacking” and “activism”—has been around since the beginning of the internet. Groups such as WikiLeaks and Anonymous believe that they are promoting freedom of information and transparency, but hacktivism can have unintended consequences and affect innocent individuals. Although frequently lauded for taking up social justice causes, hacktivism is still a form of cybercrime, and hacktivists are the ones who decide what individuals or entities should be targeted based on their own political and moral leanings, operating outside the bounds of societal or legal scrutiny. Techniques can include website defacement and mirroring, denial-of-service attacks designed to take websites offline, and exposing private or confidential information through doxing. This volume of The Reference Shelf explores recent events and controversies in the world of hacktivism, from cyberattacks by Anonymous on Russian computer systems after the invasion of Ukraine to the doxing of Supreme Court justices by pro-choice group Ruth Sent Us.